The present invention relates to current mirrors, for example, an adjustable gain range current mirror for a speakerphone system.
Current half duplex speakerphones include complementary attenuators, a transmit attenuator and a receive attenuator to control the gain in the transmit and receive paths. The gain of each path is determined by the relative strength of the signal appearing in each path. Typical half duplex speakerphones utilize a control circuit which provides a DC feedback loop to actively maintain the proper control voltages on each of the complementary attenuators such that the minimum and maximum gains (the attenuation range) of the complementary attenuators are substantially constant. Further, the difference between the minimum and maximum gains of the complimentary attenuators, which is also known as the attenuation range, is important to the overall stability of the speakerphone wherein a large attenuation range is necessary for speakerphone systems with poor sidetone or high acoustic coupling. However, for speakerphone systems with good sidetone and/or low acoustic coupling, less attenuation range is required. Furthermore, less attenuation range allows the speakerphone to switch between transmit and receive modes faster thereby better approximating a full duplex operation.
Hence, a need exists for a speakerphone system having an attenuation range control circuit which provides a plurality of gain ranges.